1. Field of the Invention
The invention generally relates to high-speed communications devices. In particular, the invention relates to interference compensation techniques in a receiver.
2. Description of the Related Art
A variety of physical impairments can limit the effective transmission of data signals over communications channels. For example, the frequency selective nature of a communication channel attenuates and phase shifts the various frequency components of an input signal differently depending on frequency. A corresponding impulse response can span several symbol intervals, which can result in time smearing and interference between successive transmitted input symbols, which is known as inter-symbol interference (ISI). The ISI resulting from the channel distortion, if left uncompensated, can undesirably result in relatively high error rates. One approach to the ISI problem is to have a receiver compensate or reduce the ISI in the received signal with an equalizer.
Within inter-symbol interference (ISI), there are post-cursor ISI and pre-cursor ISI. Past bits (bits already received) contribute to post-cursor ISI, and future bits (bits yet to be received) contribute to pre-cursor ISI.
A pulse response of the system can be used to show the ISI contributions of a system. A sample of such a pulse response is shown in FIG. 1. The pre-cursor portion 102 of the pulse response contributes to the pre-cursor ISI of earlier-in-time symbols, and the post-cursor portion 104 of the pulse response contributes to post-cursor ISI of later-in-time symbols.
A number of techniques exist to reduce the ISI of a system. A common approach to this problem is a decision feedback equalizer (DFE). Within this disclosure, a DFE refers to the feedback portion of a decision-based equalization scheme and does not include feed-forward filtering.
A typical DFE operates by determining the symbols or bits incoming data signal and using the determined symbols or bits bit multiplied by an adapted gain factor (such as in a finite impulse response filter) to cancel out the post-cursor ISI of a channel. Since a conventional DFE only has access to observed data bits and not to future data bits, a conventional DFE is unable to compensate for the pre-cursor contribution of ISI.
However, the pre-cursor contribution of ISI of a channel can have a significant impact on the error performance of the channel. Accordingly, it is desirable to reduce the overall pre-cursor noise contribution.
As illustrated in FIG. 2, a traditional approach to removing pre-cursor ISI is to use a feed-forward equalizer 202. A feed-forward equalizer 202 combined with a decision feedback equalizer (DFE) 204 reduces the pre-cursor ISI and the post-cursor ISI. With this system, the signal is passed first through a feed-forward equalizer 202 that typically includes the cancellation of the pre-cursor ISI contributions. The signal is then sent through the DFE 204, which removes the post-cursor portion of the noise.
There are a number of disadvantages associated with a feed-forward equalizer approach. A linear feed-forward equalizer that is adaptive can be difficult to implement in practice for high-speed implementations. An adaptive filter is typically desirable since the characteristics of a transmission medium can change over time. Another typical disadvantage with feed-forward equalizers is that the equalization applied to reduce the ISI is also applied to cross-talk and other noise sources. This is undesirable, since the noise can be amplified by the equalization process.
Pre-cursor ISI can also be reduced from the transmitting end of the communication system. The pre-cursor ISI can be reduced by manipulating the transmitted pulse to compensate for the distortions of the channel. This technique is known as pre-cursor pre-emphasis and is illustrated in FIG. 3. FIG. 3 illustrates a transmitted pulse for a system that compensates in the transmitter for pre-cursor ISI. One disadvantage is that the magnitude of the compensation that should be used is not known to the transmitter by itself. To make that a pre-cursor pre-emphasis system adaptable, a provision for a return communication path from the receiver and transmitter must be provided so that the channel response as observed by the receiver can be provided to the transmitter.
Another conventional approach is illustrated in U.S. Pat. No. 6,697,423 for the reduction of pre-cursor ISI in a 4-PAM or 2 Binary 1 Quaternary (2B1Q) system.
A dual DFE is another example of a conventional approach. In a dual-DFE approach, multiple decision feedback equalizers are used in parallel.